We have found that the syngeneic hosts' immune responses in the spleen to the 13762A mammary adenocarcinoma is cyclic. An initial cytolytic cell response, peaking approximately 7 days after tumor inoculation, declines between days 10-14, only to peak again on about days 18-20. Cytolytic cells in both lytic phases are T-lymphocytes. The period of depressed cytotoxic cell responses is accompanied by increased humoral blocking factors and increased numbers of splenic suppressor cells. The blocking factors are heterogeneous, consisting of tumor antigen, antigen-antibody complexes and an IgG factor appearing to be an anti-receptor site antibody. Only the latter decreases during the second wave of splenic cytotoxic cells. Investigations are proceeding on the stimuli eliciting the blocking IgG and suppressor cells.